A reduction in the nitrogen oxide (NOx) concentration is demanded for boilers, and various combustion methods are provided to respond to this demand. For example, Patent Document 1 describes a combustion method in which pulverized coal is burnt in three stages: in the first zone, the air ratio is 0.55 to 0.75 and the residence time is 0.1 to 0.3 seconds; in the second zone, the air ratio is 0.80 to 0.99 and the residence time is 0.25 to 0.5 seconds; in the third zone, the air ratio is 1.05 to 1.25 and the residence time is 0.25 to 0.5 seconds.    Patent Document 1: U.S. Pat. No. 6,325,003 (Claims, FIG. 1)